Non-contact temperature-measuring devices, as they are known in the art, usually comprise an infrared detector for receiving infrared radiation emanating from a measurement spot or zone on an object of measurement, an optical system for imaging the infrared radiation emanating from the measurement spot onto the detector, and a sighting arrangement for identifying the position and size of the measurement zone on the object of measurement. A further processing arrangement which converts the infrared detector signal into a temperature indication is also usually connected to the detector.
In many such devices, the optical system is designed so that only infrared radiation from a measurement spot or zone within the target scene or object of measurement is focused onto the detector at a given measurement distance. In most cases the measurement zone is defined as the area from which a large portion (greater than 50% and typically 90% or more) of the infrared rays focused onto the detector strike. Infrared spot detectors are useful for measuring the average temperature of the area of the measurement zone, but are generally not able to resolve separate temperature zones within the measurement zone.
The size of the measurement zone usually varies with the measurement distance from the measuring device to the target scene or object. This variation in measurement zone size can be referred to as the optical profile and generally depends upon the design of the optical system. Examples of commonly used measurement profiles include standard focus and close focus measurement profiles. For the purposes of this disclosure the term “standard focus” will refer to a measurement profile similar to that shown in FIG. 1A, i.e. where the measurement spot size is constant or increases as the distance from the objective lens increases. The term “close focus” will refer to a measurement profile similar to that of FIG. 1B, i.e. exhibiting a “bow-tie” shape. A close focus profile can be a “near focus” profile having a minimum measurement zone size at a distance of less than one half meter from the lens or a “distant focus” profile having a minimum measurement zone size at a distance of one half meter or greater from the lens. Both close focus and standard focus optical systems are useful in practical applications and non-contact temperature measurement devices are available for each and at a variety of near or distant ranges.
In applications where spot temperatures must be measured at a number of distances it may be desirable to focus the scene image in order to provide a minimal measurement zone size at multiple distances. Similarly, in applications where multiple objects of different sizes are to be measured at the same distance, it is desirable to have multiple measurement zone sizes at that distance to provide a closer fit to each object. Having both a standard focus device and a close focus device can be useful in such situations, however the added cost of a second device often precludes its purchase. In some instances, a single device with focusable optics can be used to overcome the issues of different measurement distances and object sizes, however the ability to adjust a device's optical system can significantly add to the cost of such a device. Similarly, devices which include a single detector moveable relative to the optical system in order to change the device's optical profile are often undesirable because they are more susceptible to breaking and more expensive than fixed position devices. Moreover, in many measurement applications, continuous adjustability is unnecessary because the variations in distance and object size are often discrete. Other difficulties with non-contact temperature measurement that embodiments of the invention address will become apparent throughout the following description.